Powered support assemblies which can be varied in height by means of hydraulic cylinders (hydraulic props) have been used in underground mining for decades, said powered support assemblies having, as a rule, two floor skids, a guide bar mechanism, a gob shield and a one- or multi-piece shield canopy connected to the gob shield in an articulated manner. By extension of the usually two, sometimes also four, hydraulic cylinders, the shield canopy is pressed against the “roof”, i.e. the overburden of an underground longwall face, in order to keep a chamber free in the underground formation for the arrangement of the winning machines, said chamber usually being referred to as longwall face. A plurality of shield-type supports or powered support assemblies adjustable in height form an advancing support which can be pulled forward or via which a winning installation can be pushed forward by retracting the hydraulic cylinders and pushing individual shield-type supports via substantially horizontally directed advancing cylinders which are supported against the winning installation.
The powered support assemblies or shield-type supports used in high-output winning operations comprise shield canopies whose canopy plates have lengths of five meters and above and widths of two meters and above. In this case, all the bending forces between canopy tip and canopy end or the accommodating devices for the prop heads have to be absorbed with a high degree of certainty by the supporting structure welded on below the canopy plate in order to avoid fracture of the shield canopy itself in loose or undulating formation against which the shield canopy is pressed. In order to cope with these loads, the powered support assemblies used at present usually have a box-type supporting structure having a multiplicity of longitudinal spars which consist of sheet metal strips and are stiffened via transverse plates.
In a shield-type support having a withdrawal opening, as is described, for example, in DE 198 14 246 A1, two box-section-shaped longitudinal spars are provided which extend over the entire length of the shield canopy and which at the same time form the guide device for a sliding plate in order to provide the withdrawal opening, which can be opened and closed, in the shield canopy for the withdrawal/extraction process.